Well I have a mere Yashica TLR, but I've made two attempts at a do-it-yourself filter case and I can see a third may happen some day! (This was Bay I stuff.) I began with a plastic fly box from the fishing tackle/outdoor store (Cabela's in this case), stripped the internal gizmos and cut some layers of craft foam to hold the filters reasonably snugly.
The problem is I wanted to have it also hold a lens hood, which required a deeper box. My second attempt used a two-sided container in which I drilled (with a hole saw) a circular cutout to allow the hood to occupy both sides. It worked, but the particular family of boxes has a sexy radius on all the corners so one can't fully utilize the interior space. That makes it just a little larger than I hoped it would be. Back in the 19-ought-sixties there were electronic stores that had various hinged plastic boxes to use for "projects" which were much more squared off and rectangular, but alas, I've not yet found The One. There are some shallower fly boxes that would probably work if one skipped the lens hood requirement.
Anyway, if you're into DIY projects:
(Reverse side -- black rimmed items are a Plus 1 set.)
The foam is multiple layers of craft foam, cut with an Exacto knife and glued in with double-sided tape.
It's just a bit less pocket-able than I would have liked.